Field of the Invention
The present invention relates to optical communications technologies. More specifically, it relates to an optical device that inputs and outputs optical signals to and from an optical waveguide.
Description of Related Art
In optical communications technologies, particularly, in a silicon photonic technology for forming an optical waveguide in a silicon chip, it is extremely important to optically couple a silicon optical waveguide to an external optical fiber or a source laser in an effective manner.
Examples of methods developed for optical couplings include: 1) butt coupling for directly joining a silicon optical waveguide and another optical waveguide (including an optical fiber); 2) lens coupling for optically coupling a silicon optical waveguide and another optical waveguide via a microscopic lens interposed therebetween; and 3) diffraction grating coupling for optically coupling a silicon optical waveguide and another optical waveguide by fabricating an optical diffraction grating structure in the optical waveguide on the silicon substrate.
Butt coupling requires numerical aperture (NA) conversion because two optical waveguides have different NAs and also requires high accuracy in positioning the optical waveguides. Lens coupling requires fabrication of a three-dimensional lens and also requires fixed and highly accurate positioning of components such as the lens.
A diffraction grating formed on a silicon substrate is capable of NA conversion and the structure of a diffraction grating can be fabricated with high accuracy. However, fabrication of a diffraction grating in a silicon optical waveguide enables coupling such that signal light travels in both upward and downward directions, and thus it is difficult to achieve high coupling efficiency.
FIGS. 1(a) and 1(b) schematically illustrate optical device 10 having a grating (diffraction grating) structure that takes out or outputs signal light propagated through a silicon optical waveguide to the outside or takes in or inputs signal light to the silicon optical waveguide from the outside. FIG. 1(a) is a top plan view of optical device 10 and FIG. 1(b) is a cross sectional view of optical device 10 viewed from a side. FIGS. 1(a) and 1(b) illustrate the case where signal light is taken out or output from a silicon optical waveguide to the outside.
Optical device 10 includes oxide layer 12 on silicon substrate 11 and silicon optical waveguide 13 on oxide layer 12. Optical device 10 also includes grating coupler 14 at one end of the silicon optical waveguide 13. Grating coupler 14 has approximately parallel and concentric multiple grooves. Diffracted light subjected to diffraction by grating coupler 14 exits through oxide layer 12 and silicon substrate 11.
Japanese Patent Application Publication No. 2011-107384 discloses that, in a condensing grating coupler that is disposed on a substrate and that changes an optical path of signal light emitted from an optical waveguide upward with respect to the substrate, a grating coupling unit may be formed in a layer different from a core layer of the optical waveguide, such as an upper cladding layer or another layer provided separately.
Japanese Patent Application Publication No. 2010-44290 discloses that a light reflection film made of a material such as gold is formed on an overcladding layer that covers a grating formed at an end of the optical waveguide layer.
Typically, a diffraction grating for coupling a silicon optical waveguide is formed by processing a silicon layer that is the same as the core layer of the optical waveguide. However, directly processing a core layer would markedly affect a propagation mode, causing a large propagation loss. As in the case of optical device 10 illustrated in FIGS. 1(a) and 1(b), the diffraction efficiency and the coupling efficiency of a diffraction grating formed by directly processing a core layer are concurrently determined by only the depth by which the diffraction grating is processed. It is thus not easy to design and process a diffraction grating. An object of the present invention is to accomplish optical coupling that allows the diffraction efficiency and the coupling efficiency of a diffraction grating to be independently determined. This object of the present invention includes providing an optical device that can perform such optical coupling.